Spacecraft communications systems are now widely used for communications among terrestrial sites, and for broadcast purposes.
Spacecraft are associated with large capital costs for construction, for launch, and for maintenance in orbit. As a result, it has been very important to maximize the useful or operational life of each communication spacecraft, and to maximize its utility during its period of operation. Since thrust is required to maintain a spacecraft in proper position and attitude, the spacecraft must carry propellant for providing the thrust. An important aspect of spacecraft design is that of maximizing the amount of propellant which the spacecraft carries into orbit, in order to maximize the interval during which thrust may be provided. This, in turn, requires minimization of the weight of every other portion of the spacecraft, including the communication payload.
The communication payload of a communication spacecraft commonly includes antennas, frequency converters, amplifiers, and a power system for generating electrical power from sunlight for powering the electrical equipment. As mentioned, the spacecraft is very costly. It is very important that each functional portion of the spacecraft be very reliable, so that the equipment failures do not render the communication spacecraft inoperative. In order to provide reliable operation, the various components are carefully selected, qualified for operation in space, and tested both individually and when assembled.
Communications between the communication spacecraft and the ground are by way of radio-frequency electromagnetic radiation. The operating frequency of the radiation is determined, in part, by the wavelength of the signal carriers, which in turn determines the size, and therefor the gain, of the antennas on the spacecraft. There is a tradeoff between antenna size and gain, which favors using higher frequencies. The frequencies currently used for commercial communication spacecraft are generally above 2 GHZ. While higher frequencies are better from the point of view of the antenna size and gain, there are countervailing considerations. For example, atmospheric losses or attenuation tend to be higher at frequencies much higher than 12 GHZ, and the efficacy of other electronic equipment, such as amplifiers, frequency converters, and interconnecting transmission lines, tend to be less than at lower frequencies. Thus, the operating frequency of such equipment is always a compromise of various competing factors. These factors result in operating frequencies which desirably lie between 3 and 15 GHz.
When spacecraft began to be used for communications, solid-state amplifiers were not capable of reliable operation at the required frequencies and power levels, and the travelling-wave tube (TWT), a type of vacuum tube adapted for operation at frequencies in the range of 3 to 15 GHz, were used, even though the reliability advantage of solid-state amplifiers was recognized. Solid-state amplifiers have been used to some extent, but as the technology of solid-state amplifiers has improved, the requirements placed on the amplifiers have become more severe. For example, the advent of direct-broadcast satellites has increased the power required to be delivered to the spacecraft antennas, and multichannel operation has become common. These factors, together with improvements in travelling-wave tubes, have resulted in significant use of travelling-wave tubes in present-day communication satellites, notwithstanding the recognition of their limited lifetime.
Solid-state amplifiers for operation at radio frequencies at power comparable to that of travelling-wave tubes now almost invariably use field-effect transistor (FET) amplifiers. Because of the relatively low output power of a single solid-state amplifier at the desired frequencies, a solid-state amplifier for communication use often includes paralleled FET transistor output stages, which are optimized to maximize the output power while tending to minimize distortion, and which are sufficient in number to produce, together, the desired signal output power. Because of the relatively high operating frequencies, the FETs of such amplifiers are often based on GaAs technology, which provide good high-frequency operation. In order to maximize the power gain, the FETs are operated in the common-source configuration, with the operating conditions, such as the bias conditions and the input and output impedances, adjusted to provide the desired instantaneous bandwidth necessary to accommodate the multiple carriers without too sharp a gain cutoff.
Because of the relatively low gain of a stage of solid-state amplification at the desired frequencies, a solid-state amplifier for communication use includes a cascade of stages, in which ordinarily only the last stage is a paralleled stage as described above. The distortion in a single-carrier amplifier is usually defined by gain compression. Gain compression is measured by applying an increasing amplitude of the single signal carrier to be amplified to the gate of the FET of the amplifier stage, and comparing the output power with the input power. So long as the input and output signal levels are low, the gain of such a stage tend to be at a maximum value, which is the "small-signal" gain. As the input signal level increases, the output signal level also tends to increase. The gain of the stage tends to decrease as the output signal level increases, so that the output power of the stage is less than might be expected based upon the small-signal gain. The usual measure of gain compression is made at the one-dB point, which defines an operating condition in which the output signal power is one dB, or about 25%, less than would be the case if the small-signal gain were maintained over the entire output signal range.
When a solid-state amplifier must amplify or carry multiple signal carriers, the one-dB gain compression point is no longer useful as an indicator of the operating point, because the presence of multiple carriers results in intermodulation, or mutual modulation among or between the carriers. The amount of signal power which a given amplifier can produce for each carrier in a multiple-carrier context is significantly less than that which it can produce in a single-channel context.
A solid-state amplifier for a communication spacecraft, then, includes a plurality of cascaded stages of amplification, in which at least the output stage consists of a plurality of GaAs FET stages, which are paralleled to produce the desired output power. It is well known that, because of the relatively low gain of the paralleled output a stage of a solid-state amplifier, there may be a significant contribution by the driver stage(s) to the overall distortion of the cascaded stages. A well-known arrangement for overcoming the combined distortion of the various stages of a cascaded solid-state amplifier is to interpose a distortion equalizer in the cascade. Such distortion equalizers are described, for example, by U.S. Pat. No. 5,221,908, issued Jun. 22, 1993 in the name of Katz, U.S. Pat. No. 5,146,177, issued Sep. 8, 1992 in the name of Katz et al., U.S. Pat. No. 5,038,113, issued Aug. 6, 1991 in the name of Katz et al., and U.S. Pat. No. 4,588,958, issued May 13, 1986, in the name of Katz et al.
One of the problems associated with the use of solid-state devices generally, and GaAs FETs particularly, is that of sensitivity to excessive signal level. The reliability of solid-state devices is very high, except when its ratings are exceeded. Common situations in which ratings may be exceeded are those in which the temperature of the chip rises above its rated temperature, or in which its direct operating voltages are exceeded. Because of the need to obtain the maximum signal output power from each FET of the paralleled output stage of a FET amplifier, each of the FETs is operated near its maximum allowable limits. Among the limitations is that of not exceeding the gate signal drive power. In a multicarrier context, the instantaneous peak signal power is many times the average signal power. The peak signal power P.sub.i is defined by EQU P.sub.i =2N P.sub.avg 1
where P.sub.avg is the effective or average power of the multicarrier signal. Although the peak signal power level occurs infrequently or only occasionally, it has the capability of significantly degrading the performance of the output or power stage of the FET amplifier if continued over an extended period of time. Because of the difficulty encountered in determining the exact occurrences of P.sub.i, the manufacturer specifies the maximum gate signal drive in a multicarrier context in terms of the gate current drawn. For a given type of FET transistor, the maximum average gate current which may be drawn is specified. Thus, if the multicarrier signal input to the gate of any transistor of the given type exceeds the rated value, the performance may be degraded. One way to guarantee that the limiting value of gate current is not drawn in a multicarrier context is to operate the output or power stage much below its ratings, so that the input signal level can never cause the rated gate current. This, however, is very inefficient, because the full capability of the FETs of the output stage is not utilized.
Improved FET amplifiers are desired.